An important part of the operation of a mobile station in a cellular radio system is the constant preparation for handovers. To enhance its preparedness for changing base stations, a mobile station usually performs pre-synchronization which means that it receives brief transmissions from other base stations than the one with which it is currently actively communicating. Each base station transmits these brief information bursts on a certain channel, which may be called a synchronization channel or SCH. By decoding the contents of the SCH transmissions the mobile station keeps itself aware of the availability of alternative communication resources through other base stations and the timing of their occurrence.
As an example we will discuss the known GSM or Global System for Mobile telecommunications. Each GSM base station transmits on a certain beacon frequency a pattern of common channel transmission bursts, in which an SCH burst occurs five times in a cycle of 51 frames (a frame consists of eight consecutive Burst Periods, known also as BPs or time slots). To be exact, the SCH bursts occur in the first time slot (i.e. the time slot number 0) of frames 0, 10, 20, 30 and 40. A mobile station that is in active communication with a certain base station uses a most typically a so-called full rate traffic channel meaning that it receives during one time slot per each downlink frame and transmits during one time slot per each uplink frame with the exception that once in each cycle of 26 consecutive frames there is a so-called idle frame during which the mobile station does not receive or transmit user data. As a result of the idle frame concept the mobile station has, once in every 26 frames, the opportunity to use a longer time period for measuring, receiving and decoding the synchronization signals from other base stations. Let us analyze the length of said longer time period, known also as the measurement window, in somewhat greater detail.
FIG. 1 illustrates a part of a train of downlink transmission frames and a part of the corresponding train of uplink transmission frames. In each transmission frame the numbering of time slots goes from 0 to 7. There is a numbering shift of 3 time slots between downlink and uplink so that the uplink time slot number 0 occurs simultaneously with the downlink time slot number 3, the uplink time slot number 1 occurs simultaneously with the downlink time slot number 4 and so on. Additionally there is a small alignment difference designated as α between uplink and downlink. A full-rate traffic channel is bidirectional and uses the same time slot number in both directions. The hatched frames in each direction are the idle frames during in which no transmission of user data takes place.
The length of the measurement window is independent of the time slot number used by the traffic channel, but its location in relation to the idle frame borders is not. Let us assume that time slot 3 is used. The measurement window 101 during which the mobile station does not have any activity relating to the traffic channel starts at the end of the uplink time slot 3 in the last uplink frame before the idle frame and ends at the beginning of the downlink time slot 3 in the first downlink frame after the idle frame. The theoretical maximum length of the time available for measurements equals therefore the total length of 12 successive time slots or BPs plus α. Measurement window locations for traffic channels that use other time slot numbers are easily obtained from FIG. 1 by shifting the measurement window 101 left or right by intervals of one time slot in length.
FIG. 1 also shows the shorter windows 102 and 103 that are available for the mobile station during each frame. The lengths of these shorter windows are 2 BPs−α and 4 BPs+α respectively. They are usually too short for reasonable measuring of synchronization bursts from other base stations. We must remember that propagation delays, the settling times of tunable components and other sources of error in timing make the actual available length of the measurement windows shorter than the theoretical maximum.
The occurrence of an idle frame once in every 26 frames on one hand and the transmission of SCH bursts in periods of 51 frames on the other hand guarantee that even if a mobile station would not be able to catch an SCH burst from a certain base station in a certain measurement window due to misaligned timing, it will be able to do so during some of the following measurement windows: the numbers 26 and 51 have no common divisors greater than one. However, it may take as long as 11 times 26 frame durations before the next opportunity arrives. In some cases this delay may seriously degrade pre-synchronization effectiveness.
The situation may be even worse in such systems where there is an attempt to align the measurement windows to the occurrence of SCH bursts from neighboring base stations. For example in a proposed enhancement to the existing GSM it has been suggested that both the packet data channels and the control channels relating to packet data services should be based on a multiframe structure of 52 consecutive transmission frames. The control channel timing should be aligned with that of traffic channels so that for example the SCH transmissions should all take place during the idle frames in the traffic channels. Let us analyze the resulting relations between SCH transmissions and measurement windows depending on which traffic channel slot the mobile station is using. For simplicity we will again assume that a bi-directional full rate traffic channel is concerned.
FIG. 2a illustrates the arrangement of five partially overlapping cells 201, 202, 203, 204 and 205. In each cell there is a base station BTS. In each of the cells 201, 202, 203 and 204 there is also a mobile station 206, 207, 208 and 209. For each of these mobile stations cell 205 (among others) is a candidate for handover, so they must try to pre-synchronize themselves to the base station of cell 205 by receiving its SCH transmissions. We will first assume that all base stations in FIG. 2a apply synchronized frame timing so that the idle frames are simultaneous for all mobile stations.
FIG. 2b illustrates the possible measurement window occurrences for mobile stations 206, 207, 208 and 209 depending on the time slot used for the traffic channel for each of them. The vertical hatched columns 210, 211, 212 and 213 illustrate four alternative transmission times of an SCH burst from the base station of cell 205. These alternative transmission times correspond to the time slots 1, 3, 5 and 7 of the synchronized downlink idle frames. Naturally the transmission time of the SCH burst could as well be selected to coincide with any of the time slots 0, 2, 4 or 6 of the synchronized downlink idle frames. For each mobile station eight possible measurement window locations 206A to 206H, 207A to 207H, 208A to 208H and 209A to 209H are shown.
We may expect that if the measurement window starts or ends exactly simultaneously with the start or end of the SCH transmission time respectively, it is not possible to use the SCH signal for pre-synchronization. If the SCH transmission time is within the measurement window but closer than one time slot width from its border, it is still doubtful whether the pre-synchronization will be successful. Only if the SCH transmission time is well within the measurement window the pre-synchronization will succeed. Based on this assumption and the teachings of FIG. 2b we may formulate the following table:
TABLE 1Pre-synchronization toTime slot # ofSCH burst coincident with idle time slot #traffic ch.13570XXX1XXX?2XXXX3XXXX4XXXX5?XXX6XXX7?XX
Here X means successful pre-synchronization, ? means unreliable pre-synchronization and an empty box means no pre-synchronization at all. For example we may note that if the SCH transmission is synchronized to be coincident with the time slot 1 of the downlink idle frame, those mobile stations using one of the time slots from 0 to 4 for their traffic channel will be able to pre-synchronize, a mobile station using time slots 5 for its traffic channel will be only possibly able to pre-synchronize, and those mobile stations using one of the time slots 6 or 7 for their traffic channel will not be able to pre-synchronize.
The example shown in FIG. 2b relies on the above-mentioned assumption that the mobile stations 206, 207, 208 and 209 have simultaneous idle frames. FIG. 2c illustrates an alternative situation known as synchronized shifted idle frames. The frame cycles of the base stations in cells 201, 202, 203 and 204 are synchronized to each other with a shift of two time slots from cell to cell. In other words, the idle frame of mobile station 207 begins two time slots later than that of mobile station 206, the idle frame of mobile station 208 begins further two slots later and so on.
The reference designators in FIG. 2c are the same as in FIG. 2b, because only the mutual timing of the idle frames is changed.
The mobile station designated as 206 has the best chance for successful presynchronization. The successive time shifts concerning the other mobile stations mean that the mobile station designated as 209 has only very modest possibilities for successful presynchronization: only the SCH burst transmitting times 212 and 213 are possible, and even then the mobile station 209 must have some of the very first time slots in a frame allocated for its full-rate traffic channel.
There exists a prior art proposal of reducing the uncertainty in successful presynchronization by lengthening the measurement window of each mobile station. In practice this means that the transmission of payload data is forbidden for the continuous duration of more than one idle frame at each time. A longer measurement window certainly increases the chances of a certain SCH burst to be received, but simultaneously it leaves a smaller relative amount of radio resources to the transmission of payload data, making it difficult to maintain a circuit-switched connection.